Lasers

ABSTRACT

A laser has an optical laser cavity one end of which is constituted by a partial reflector formed in optical waveguide by an unbalanced Michelson interferometer comprising an optical waveguide splitter/combiner (23; 33) and two optical waveguide Bragg grating reflectors (24a, 24b; 34a, 34b; 44a, 44b).

BACKGROUND OF THE INVENTION

This invention relates to lasers, and in particular to a class of laserhaving an optical laser cavity one end of which is formed in opticalwaveguide, typically optical fibre waveguide. One example of such alaser is an optically pumped erbium doped optical fibre laser providedwith optical fibre Bragg grating reflectors to define the ends of itsoptical cavity. Another example, depicted in FIG. 1, is constituted by asemiconductor injection laser diode chip 10 provided with ananti-reflection coating 11 on one end facet 12 to which is opticallycoupled a length of optical fibre pigtail 13 in which there is a Bragggrating reflector 14 defining one end of a laser optical cavity whoseother end is provided by the end facet 15 of the injection laser chipremote from the anti-reflection coated end facet. Interest in such diodelasers, known as external cavity diode lasers arises at least in part tothe fact the Bragg grating reflector provides a means of locking thelaser frequency.

SUMMARY OF THE INVENTION

The present invention is directed to a modified configuration ofreflector which can be employed to provide improved laser performance byremoval of out-of-band spontaneous emission from the output of laser orby enhanced inhibition of mode hopping.

According to the present invention there is provided a laser having anoptical laser cavity one end of which is constituted by a partialreflector formed in optical waveguide by an unbalanced Michelsoninterferometer comprising an optical waveguide splitter/combiner and twooptical waveguide Bragg grating reflectors.

An additional advantageous feature of this arrangement is that thereflectivity of the partial reflector can be readily trimmed for optimumpower output of the laser by using the photo-refractive effect to alterthe optical path length of one of the interference arms of theMichelson.

BRIEF DESCRIPTION OF THE DRAWINGS

There follows a description of lasers embodying the invention inpreferred forms.

FIG. 1 (to which previous reference has already been made) schematicallydepicts a prior art external cavity diode laser, and FIGS. 2, 3 and 4schematically depict external cavity diode lasers with alternative formsof imbalance Michelson interferometer type external Bragg grating typereflector.

DETAILED DESCRIPTION OF THE DRAWINGS

The lasers of FIGS. 2, 3 and 4 have the same semiconductor laser chipcomponents as the laser of FIG. 1, and so these components have beenidentified in these Figures using the same index numerals as thoseemployed in FIG. 1. In each instance the place of the fibre pigtail 13and single Bragg grating reflector 14 has been taken by an unbalancedMichelson interferometer. In the case of the laser of FIG. 2, thisunbalanced Michelson interferometer has matched Bragg grating reflectors24a and 24b located equidistant from the coupling region 25 of a 2×2waveguide splitter/combiner 23 having ports, A, B, C and D. Port A isoptically coupled with the anti-reflection coated facet 12 of the diodechip 10. The two Bragg grating reflectors 24a and 24b are respectivelyformed in the two arms of the splitter/combiner that respectivelyterminate in ports C and D. Port B constitutes the optical output of thelaser.

If the splitter combiner 23 were to have been a balanced 3 dBsplitter/combiner, then light launched into the splitter/combiner by wayof port A would have been divided by its coupling region 25 equallybetween the two limbs terminating in ports C and D. Light within thewavebands of Bragg grating reflectors 24a and 24b would be reflectedback to the coupling region 25 where, because of the equidistance of thereflectors 24a and 24b from the coupling region, all this light wouldemerge from the splitter/combiner by way of port 8. This means that this(balanced) Michelson interferometer would fail to provide any feedback,and so would not function as one of the end of a laser optical cavity.Thus a laser would not be formed. It is for this reason that some formof imbalance in the Michelson interferometer is required.

In the case of the laser of FIG. 2 this imbalance is provided by makingthe splitter/combiner deliberately unbalanced, directing more of theoptical power launched into it by way of port A into one of the two armsrespectively terminating in ports C and D than into the other. Thismeans that the amplitude of the light reflected by Bragg grating coupler24a back into the coupling region 25 is not equal to that reflected backinto it by Bragg grating coupler 24b. In consequence some of thereflected light emerges from this splitter/combiner by way of port A andhence provides the feedback necessary for laser action. Typically thefeedback may be about 10%, leaving about 90% to emerge from port B,which constitutes the output port of the laser.

One feature to be particularly noted contrasting the laser of FIG. 2from that of FIG. 1 is that spontaneous emission launched into thesplitter/combiner from the diode chip 10 is, with the exception of thatlying within the reflection band of the Bragg grating reflectors 24a and24b, separated from the laser output to emerge from ports C and D ratherthan the laser output port B. No such separation occurs in the case ofthe laser of FIG. 1. Typically the Bragg grating reflectors are createdin photosensitive optical waveguide, in which case the Q of the lasercavity can be adjusted by using UV light to trim one of the arms of theMichelson interferometer to add to its unbalance.

An alternative form of Michelson interferometer imbalance is employed inthe laser of FIG. 3. Like the laser of FIG. 2, this laser has one end ofits laser optical cavity formed by an unbalanced Michelsoninterferometer comprising a waveguide splitter/combiner 33 and twooptical waveguide Bragg grating reflectors 34a and 34b. The reflectors34a and 34b are identical with reflectors 24a and 24b of the laser ofFIG. 2, but are located at different distances from the coupling region35 of splitter/combiner 33. Splitter/combiner 33 is distinguished fromsplitter/combiner 23 of FIG. 2 in that splitter/combiner 33 is abalanced 3 dB splitter/combiner.

A prior art method of adjusting the Q of an external cavity diode laserso as to optimise its output has involved replacing its external mirrorwith a different mirror of a different reflectivity. Such an approach iscostly in both time and resources and is limited by the step sizesbetween the reflectivities of the mirrors used. With the laserconfiguration of FIG. 3, by making use of the photosensitivity ofwaveguides, the Q is much more readily adjusted, and moreover, suchadjustment can be achieved during operation of the laser. Suchadjustment may be performed by irradiating with UV light one of thewaveguides in a region between its Bragg grating and the coupling regionof the splitter/combiner. Such irradiation is employed to alter theeffective refractive index of the waveguide in that region, and hencethe optical path length of that arm of the Michelson interferometer,thereby changing the reflectivity presented to the diode chip by theinterferometer.

In the case of the laser of FIG. 3 it is expected that the unequaldistances of the two grating reflectors from the coupling region 35 canbe arranged to provide a measure of inhibition of mode-hopping. With theappropriate relative displacement, when the laser is in stableoperation, the unbalanced Michelson interferometer functions to lightwithin the laser cavity as a stable low reflectivity reflector. If thereis a tendency to mode hop such mode hopping would produced areflectivity change since the phase upon reflection from the gratings ischanged, and therefore the condition of interference back at the couplerchanges. This reflectivity change is sensed at the laser diode chip 10,changing the carrier density and thus its effective refractive index andeffective optical path length so as to bring the laser emission back toits original wavelength. Like the laser of FIG. 2, this laser separatesout-of-band spontaneous emission noise from the output of the laserprinted at port A.

A variant of the laser of FIG. 3 is depicted in FIG. 4. This uses thesame splitter/combiner 33 as the laser of FIG. 3 but, instead of havingdisplaced but otherwise matching Bragg grating reflectors 34a and 34b,provides imbalance by the use of dissimilar chirped Bragg gratingreflectors 44a and 44b. These are of different length and are chirped atdifferent rates in such a way as to provide the same bandwidth for eachgrating. When there is a change of effective optical path distancebetween the reflecting facet 15 of the laser diode chip 10 and thecoupling region 35 of splitter/combiner 33 that tends to promote achange of emission frequency, then the reflection point in the longergrating will tend to move further than that in the shorter gratingthereby providing a compensating phase shift.

We claim:
 1. A laser having an optical laser cavity extending betweenfirst and second reflecting optical laser cavity ends, and havingextending between said reflecting ends a substantially non-reflectingtransmission path, wherein said first end is constituted by a partialreflector formed in optical waveguide by an unbalanced Michelsoninterferometer comprising an optical waveguide splitter/combiner and twooptical waveguide Bragg grating reflectors, and wherein optical gain inthe laser is provided by a length of optically amplifying waveguide. 2.A laser as claimed in claim 1, wherein the optical waveguide Bragggrating reflectors are optical fibre optical waveguide reflectors.
 3. Alaser as claimed in claim 2, wherein the optical waveguidespitter/combiner is an optical fibre optical waveguidesplitter/combiner.
 4. A laser having an optical laser cavity extendingbetween first and second reflecting optical laser cavity ends, andhaving extending between said reflecting ends a substantiallynon-reflecting transmission path, wherein said first end is constitutedby a partial reflector formed in optical waveguide by an unbalancedMichelson interferometer comprising an optical waveguidesplitter/combiner and two optical waveguide Bragg grating reflectors,and wherein the optical waveguide Bragg grating reflectors are chirpedgratings of different rates of chirp but the same bandwidth.
 5. A laseras claimed in claim 4, wherein optical gain in the laser is provided bya diode laser chip.